A half-wave shaping multilevel AC converter topology and control method thereof

Abstract

The present invention discloses a half-wave shaping multilevel AC converter topology and control method suitable for AC-AC frequency conversion fields such as low-frequency power transmission. The converter comprises three phase units, each of which is connected to symmetrical converters on the 1st and 2nd sides via a DAHB isolation unit. Both converters consist of a multilevel shaping unit and a half-wave commutation unit, and are connected to AC systems of different frequencies. The multilevel shaping unit is composed of a cascade of half-bridge submodules, with the submodule capacitors consisting of two capacitors connected in series. The submodules and the DAHB isolation unit share a DC capacitor, which helps reduce the number of capacitors and improve power density. The half-wave commutation unit is a full-bridge circuit, with the bridge arm switches consisting of multiple devices connected in series, which are turned on and off according to the fundamental frequency cycle, flipping the multilevel sinusoidal half-waves modulated by the multilevel shaping unit into full sinusoidal waves. The control method for low-frequency power transmission of the present invention is as follows: the power frequency side adopts fixed module capacitor voltage control, the low-frequency side adopts fixed AC voltage control, and the DAHB adopts fixed low-frequency side module capacitor voltage control.

Description

Technical Field

[0001] The present invention relates to the technical field of converters, and in particular to a half-wave shaping multi-level AC converter topology and a control method thereof. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] High-voltage, large-capacity AC exchangers are the core equipment of systems such as flexible low-frequency power transmission, low-frequency interconnection of urban power grids, variable-speed pumped storage, and low-frequency aggregation of new energy.

[0004] Existing high-voltage, high-capacity AC converters mostly use cascaded modular multilevel technology based on full-sine wave modulation conversion, such as the back-to-back modular multilevel converter (BTB-MMC) and the modular multilevel matrix converter (M3C). These converters use a large number of submodules and capacitors, resulting in low power density and poor economic efficiency. M3C also faces the problem of complex decoupling control. Improving the power density of high-voltage, high-capacity AC converters, reducing the number of submodules and capacitor capacity, and reducing control complexity are technical challenges that need to be addressed. Summary of the Invention

[0005] To solve the above problems, the present invention proposes a half-wave shaping multi-level AC converter topology and a control method thereof. The multi-level shaping unit generates a sinusoidal half-wave voltage, and only the number of sub-modules required for half-wave modulation needs to be configured, which greatly reduces the number of sub-modules. A half-wave commutation unit is introduced to flip the multi-level sinusoidal half-wave into a complete multi-level sinusoidal wave output. A DAHB isolation unit is introduced, and a high-frequency transformer is used to achieve voltage conversion and electrical isolation. The DAHB isolation unit circuit is nested and combined with the sub-modules of the multi-level shaping unit, sharing a series DC capacitor, which is conducive to further reducing the number of capacitors and devices and improving power density.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a half-wave shaping multilevel AC converter topology, comprising: three phase unit circuits of identical structure, each phase unit circuit connected to a symmetrically structured 1-side and 2-side converter via a dual active half-bridge (DAHB) isolation unit, the converters on both sides being connected to AC systems of different frequencies;

[0008] The converters on both sides 1 and 2 include a multi-level shaping unit and a half-wave commutation unit. The multi-level shaping unit is formed by cascading half-bridge submodules, and the submodule capacitors are in the form of two capacitors connected in series. The half-wave commutation unit is a full-bridge (H-bridge) circuit structure, and its four bridge arm switches are each composed of multiple IGBTs (or IGCTs) connected in series. The diagonal bridge arm switches form a group, and the two groups of switches are alternately turned on and off according to the fundamental frequency cycle.

[0009] The DAHB isolation unit consists of two half-bridge circuits and a high-frequency transformer. The two half-bridge circuits and the multi-level shaping unit sub-modules on both sides share a series DC capacitor. The two ends of the primary / secondary winding of the high-frequency transformer are respectively connected to the midpoint of the sub-module series capacitor and the midpoint of the half-bridge circuit bridge arm.

[0010] According to one embodiment of the present invention, the converters on side 1 and side 2 connected to AC systems of different frequencies need to be connected through a DC-DC isolation stage to achieve flexible conversion of system voltage, power, and frequency and electrical isolation.

[0011] According to one embodiment of the present invention, the DC-DC isolation stage adopts a DAHB circuit and a high-frequency transformer. The sub-modules of the multi-level shaping unit are nested and combined with the DAHB isolation unit, sharing a series DC capacitor. The two ends of the series capacitor provide DC access ports for the DAHB isolation unit, and the midpoint of the series capacitor provides an AC access point for the primary (secondary) winding of the high-frequency transformer of the DAHB isolation unit. The other end of the winding is connected to the midpoint of the DAHB half-bridge circuit, which is conducive to reducing the number of capacitors and devices and improving power density.

[0012] According to one embodiment of the present invention, the number of multi-level shaping unit submodules and the number of DAHB isolation unit modules on both sides are the same, set to N, and the number of modules N can be configured according to the half-wave peak range of the AC voltage of the system on both sides and the submodule capacitor voltage: N = V m / (M·V dcm ), where V m is the AC phase voltage amplitude, M is the rated modulation ratio, V dcm is the submodule capacitor voltage.

[0013] According to one embodiment of the present invention, the multi-level shaping units on side 1 and side 2 respectively use a period (half-wave period) corresponding to twice the frequency of the AC system on their respective sides as a modulation period to generate a multi-level sinusoidal half-wave voltage corresponding to twice the frequency of the system;

[0014] The half-wave commutation units on side 1 and side 2 flip the multi-level sinusoidal half waves on both sides into complete multi-level sinusoidal wave voltages, which are filtered by the filter inductors (or other filter elements) on both sides and then connected to the AC systems on both sides.

[0015] According to one embodiment of the present invention, the half-wave commutation unit flips the multi-level sinusoidal half-wave according to the sinusoidal reference waves of the phase voltages on both sides, and obtains a complete multi-level sinusoidal wave on the AC side:

[0016] When the phase voltage sinusoidal reference wave is positive, the commutation switches Q1 and Q4 are turned on, and the AC side outputs the positive half-wave of the multi-level sinusoidal voltage;

[0017] When the phase voltage sinusoidal reference wave is negative, the commutation switches Q2 and Q3 are turned on, and the AC side outputs the negative half-wave of the multi-level sinusoidal voltage.

[0018] In a second aspect, a control method for a half-wave shaping multilevel AC converter topology applicable to the field of low-frequency power transmission includes:

[0019] The multi-level shaping units on both sides of the power frequency / low frequency use a half-sine wave as the modulation period to generate a multi-level half-sine wave corresponding to twice the system frequency. The half-wave commutation unit flips the half-sine wave into a complete multi-level sine wave, which is then connected to the AC system on both sides through the filter element.

[0020] The power frequency side uses double closed-loop control to realize the average voltage control and reactive power control of the sub-module capacitors on the fixed power frequency side. The low frequency side uses double closed-loop control to realize the AC voltage control of the fixed low frequency side. The DAHB isolation unit uses phase shift control to realize the average voltage control of the sub-module capacitors on the fixed low frequency side.

[0021] The beneficial effects of the present invention are:

[0022] The present invention proposes a novel half-wave shaping multi-level AC converter topology, which is suitable for AC-AC conversion scenarios such as flexible low-frequency power transmission. The multi-level shaping unit only needs to generate a sinusoidal half-wave voltage, significantly reducing the number of configured sub-modules. The half-wave commutation unit flips the multi-level sinusoidal half-wave into a complete multi-level sinusoidal wave output; the DAHB isolation unit uses a high-frequency transformer to achieve voltage conversion and electrical isolation, and the DAHB isolation unit circuit is nested and combined with the sub-modules of the multi-level shaping unit, sharing a series DC capacitor, which is conducive to further reducing the number of capacitors and components, and improving power density and economy.

[0023] The present invention proposes a half-wave shaping multilevel AC converter topology and a control method thereof. The DC link and high-frequency transformer of the DAHB isolation unit can frequency-isolate the AC systems on both sides while transmitting power, avoiding direct coupling of electrical quantities of the heterogeneous frequency systems on both sides, which is beneficial to reducing the complexity of the converter decoupling control. The control method can achieve stable control of the average voltage of the module capacitors on the power frequency side and the low frequency side, reactive power control on the power frequency side, and AC voltage control on the low frequency side. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1A structural diagram of a half-wave shaping multi-level AC converter topology provided by the present invention;

[0025] Figure 2 This is a structural diagram of the submodule of the multi-level shaping unit provided by the present invention;

[0026] Figure 3 A structural diagram of a half-wave commutation unit provided by the present invention;

[0027] Figure 4 A structural diagram of the nested combination of the multi-level shaping unit submodule and the DAHB isolation unit provided by the present invention;

[0028] Figure 5 A schematic diagram of half-wave shaping provided by the present invention;

[0029] Figure 6 The power frequency side control method provided by the embodiment of the present invention;

[0030] Figure 7 A control method for a DAHB isolation unit provided in an embodiment of the present invention;

[0031] Figure 8 A low-frequency side control method provided by an embodiment of the present invention;

[0032] Figure 9 The low-frequency side multi-level half-sine wave voltage and full multi-level sinusoidal wave voltage provided by the embodiments of the present invention;

[0033] Figure 10 The power frequency side multi-level half-sine wave voltage and full multi-level sinusoidal wave voltage provided by the embodiment of the present invention;

[0034] Figure 11 The submodule capacitor voltage waveform of the low-frequency side multi-level shaping unit provided by the embodiment of the present invention;

[0035] Figure 12 The submodule capacitor voltage waveform of the power frequency side multi-level shaping unit provided in an embodiment of the present invention;

[0036] Figure 13 The AC voltage waveform on the low-frequency side of the converter provided by the embodiment of the present invention;

[0037] Figure 14 The AC current waveform on the low-frequency side of the converter provided by the embodiment of the present invention;

[0038] Figure 15 The AC voltage waveform of the power frequency side grid provided by the embodiment of the present invention;

[0039] Figure 16 The AC current waveform of the power frequency side grid provided by the embodiment of the present invention; DETAILED DESCRIPTION

[0040] The present invention is further described in detail below with reference to the accompanying drawings and examples.

[0041] It should be noted that the specific implementation methods described herein are only used to explain the relevant content and are not intended to limit the present invention. It should be noted that the implementation methods and features of the implementation methods in the present invention can be combined with each other without conflict.

[0042] This embodiment provides a novel half-wave shaping multilevel AC / DC converter topology suitable for AC / DC frequency conversion scenarios, such as offshore low-frequency power transmission. An offshore wind farm outputs low-frequency power through a wind farm control system, which is then transmitted via a 20Hz submarine cable to the low-frequency side (system 1) of the half-wave shaping multilevel AC / DC converter provided in this embodiment. The power-frequency side (system 2) of the half-wave shaping multilevel AC / DC converter is connected to the 50Hz grid.

[0043] This embodiment provides a half-wave shaping multi-level AC converter topology, such as Figure 1 As shown, it includes: three phase unit circuits with the same structure, each phase unit circuit is connected to the symmetrical 1-side and 2-side converters through a DAHB isolation unit, and the converters on both sides are respectively connected to AC systems with different frequencies;

[0044] Among them, the converters on side 1 and side 2 both include a multi-level shaping unit and a half-wave commutation unit. The multi-level shaping unit is formed by cascading half-bridge submodules, and the submodule capacitors are in the form of two capacitors connected in series, such as Figure 2 As shown, the half-wave commutation unit is a full-bridge (H-bridge) circuit structure, as shown in Figure 3 As shown, the four bridge arm switches are composed of multiple IGBTs (or IGCTs) connected in series, the diagonal bridge arm switches form a group, and the two groups of switches are alternately turned on and off according to the fundamental frequency cycle;

[0045] In this embodiment, the DAHB isolation unit is composed of two-side half-bridge circuits and a high-frequency transformer. The submodules of the multi-level shaping unit are nested and combined with the DAHB isolation unit, sharing a series DC capacitor. Figure 4 As shown, the two ends of the series capacitor provide DC access ports for the DAHB isolation unit, and the midpoint of the series capacitor provides an AC access point for the primary (secondary) winding of the DAHB isolation unit's high-frequency transformer. The other end of the primary (secondary) winding is connected to the midpoint of the DAHB half-bridge circuit, which is beneficial to reducing the number of capacitors and components and improving power density.

[0046] It should be noted that the number of multi-level shaping unit submodules and the number of DAHB isolation unit modules on both sides are the same, set to N. The number of modules N can be configured according to the half-wave peak range of the AC voltage on both sides and the submodule capacitor voltage: N = V m / (M·V dcm ), where Vm is the AC phase voltage amplitude, M is the rated modulation ratio, V dcm is the submodule capacitor voltage.

[0047] The multi-level shaping units on the 1st and 2nd sides of the converter use the period (half-wave period) corresponding to twice the frequency of the AC system on their respective sides as the modulation period, generating a multi-level sinusoidal half-wave voltage corresponding to twice the system frequency;

[0048] The half-wave commutation units on side 1 and side 2 convert the multi-level sinusoidal half-waves on both sides into full multi-level sinusoidal wave voltages, such as Figure 5 As shown, after being filtered by filter inductors (or other filter elements) on both sides, the signals are connected to the AC systems on both sides.

[0049] The half-wave commutation unit flips the multi-level sinusoidal half-wave according to the sinusoidal reference waves of the phase voltages on both sides, and obtains a complete multi-level sinusoidal wave on the AC side:

[0050] When the phase voltage sinusoidal reference wave is positive, the commutation switches Q1 and Q4 are turned on, and the AC side outputs the positive half-wave of the multi-level sinusoidal voltage;

[0051] When the phase voltage sinusoidal reference wave is negative, the commutation switches Q2 and Q3 are turned on, and the AC side outputs the negative half-wave of the multi-level sinusoidal voltage.

[0052] In one embodiment, the low frequency side adopts constant AC voltage control, such as Figure 6 As shown, specifically:

[0053] Low-frequency side d-axis voltage command value v gd1ref and the low-frequency side d-axis voltage v gd1 Subtract and output the low-frequency side d-axis current command value i through PI control d1ref； Low-frequency side d-axis current command value i d1ref and the low-frequency side d-axis current i d l is subtracted, and after PI control, the negative value is obtained and the low-frequency side d-axis voltage v gd1 Addition and decoupling quantity ω1L1i q1 Add together to get the low-frequency side d-axis reference voltage u d1 ; Similarly, we get the q-axis reference voltage u q1 ;u d1 with u q1 After dq / abc transformation, the three-phase sinusoidal modulation wave on the low-frequency side is obtained. The half-sine modulation wave of each phase multi-level shaping unit on the low-frequency side is obtained through the half-wave modulation wave generation algorithm. Then, the multi-level modulation strategy is adopted to generate the trigger pulse of the multi-level shaping unit on the low-frequency side; the three-phase sinusoidal modulation wave on the low-frequency side is subjected to the commutation modulation algorithm to generate the trigger pulse of the half-wave commutation unit on the low-frequency side, which controls the alternating conduction of its two groups of bridge arm switches Q1 and Q4, and Q2 and Q3.

[0054] The DAHB isolation unit uses a fixed low-frequency side submodule capacitor average voltage control, such as Figure 7 As shown, specifically:

[0055] Average value of capacitor voltage of submodule of three-phase multi-level shaping unit on low-frequency side With the instruction value v dc1ref After making the difference, the phase shift ratio D1 is obtained through PI control, and the DAHB low-frequency side half-bridge trigger pulse and the power frequency side half-bridge trigger pulse are obtained through the single phase shift modulation algorithm.

[0056] The power frequency side adopts the fixed power frequency side submodule capacitor average voltage control and reactive power control, such as Figure 8 As shown, specifically:

[0057] Average value of capacitor voltage of three-phase multi-level shaping unit submodule on power frequency side With the instruction value v dc2ref After the difference is made, the PI control is used to obtain the power frequency side d-axis current command value i d2ref , the power frequency side d-axis current command value i d2ref Subtract the power frequency side d-axis current id2 and after PI control, add it to the power frequency side d-axis voltage v gd2 Addition and decoupling quantity ω2L2i q2 Subtract and get the power frequency side d-axis reference voltage u d2 ; Power frequency side q axis current command value i q2ref and the power frequency side q-axis current i q2 Subtract, and after PI control, it is equal to the power frequency side q axis voltage v gq2 Addition and decoupling quantity ω2L2i d2 Add them together to get the power frequency side q-axis reference voltage u q2 ;u d2 with u q2 After dq / uvw transformation, the three-phase sinusoidal modulation wave on the power frequency side is obtained. The half-wave modulation wave generation algorithm is used to obtain the half-sine modulation wave of the multi-level shaping unit of each phase on the power frequency side. Then, the multi-level modulation strategy is adopted to generate the trigger pulse of the multi-level shaping unit on the power frequency side. The three-phase sinusoidal modulation wave on the power frequency side is subjected to the commutation modulation algorithm to generate the trigger pulse of the half-wave commutation unit on the power frequency side, which controls the alternating conduction of its two groups of bridge arm switches Q1 and Q4, and Q2 and Q3.

[0058] Use Matlab / Simulink platform to build Figure 1 In the system simulation model shown, the AC power supply voltage on the low-frequency side increases by 2% at 0.5s to simulate the increase in the output of the low-frequency side power supply. Figure 9 and Figure 10 The simulation results of multi-level half-sine wave voltage and full multi-level sinusoidal wave voltage in the 0.4-0.5s time period are given respectively, which are consistent with the working principle. Figure 11and Figure 12 The simulation results of the submodule capacitor voltage of the multilevel shaping unit on the low-frequency side and the power frequency side are given respectively. Under the control of DHAB fixed low-frequency side submodule capacitor average voltage, power frequency side fixed power frequency side submodule capacitor average voltage control and reactive power control, the capacitor voltage remains constant. Figure 13-16 The simulation results of the AC voltage and AC current on the low-frequency side and the power frequency side are given respectively. Under the control of the constant AC voltage on the low-frequency side, the input power increases, the AC voltage on the low-frequency side of the converter remains constant, and the current increases. Under the control of the average voltage of the power-frequency sub-module capacitor and the reactive power control on the power-frequency side, the current on the power-frequency side increases and the output power increases. The simulation results prove the effectiveness of the present invention.

[0059] The foregoing is merely a description of the embodiments of the present invention and is not intended to limit the present invention. It will be apparent to those skilled in the art that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of the claims of the present invention.

Claims

1. A half-wave shaping multi-level AC converter topology, characterized in that: include: Three identical phase unit circuits, each connected to symmetrically structured converters on the 1st and 2nd sides via a dual-active-half-bridge (DAHB) isolation unit. The converters on both sides are connected to AC systems of different frequencies. Wherein, the converters on side 1 and side 2 both include a multi-level shaping unit and a half-wave commutation unit. The multi-level shaping unit is formed by cascading half-bridge submodules, and the submodule capacitors are in the form of two capacitors connected in series. The half-wave commutation unit is a full-bridge (H-bridge) circuit structure, and its four bridge arm switches are each composed of multiple IGBTs or IGCTs connected in series. The diagonal bridge arm switches form a group, and the two groups of switches are alternately turned on and off according to the fundamental frequency cycle. Among them, the IGBT or IGCT whose emitter is connected to the inductor is Q1, the IGBT or IGCT connected to Q1 is Q3, the IGBT or IGCT whose collector is connected to the inductor is Q2, and the IGBT or IGCT connected to Q2 is Q4; The DAHB isolation unit consists of two half-bridge circuits and a high-frequency transformer. The two half-bridge circuits and the multi-level shaping unit sub-modules on both sides share a series DC capacitor. The primary and secondary windings of the high-frequency transformer are connected to the midpoint of the sub-module series capacitor and the midpoint of the half-bridge circuit bridge arm respectively. The DC-DC isolation stage uses a DAHB circuit. The submodules of the multi-level shaping unit are nested and combined with the DAHB isolation unit, sharing a series DC capacitor. The midpoint of the series capacitor provides an AC access point for the DAHB isolation unit, which helps reduce the number of capacitors and components and improve power density. The multi-level shaping units on side 1 and side 2 respectively use the period corresponding to the double frequency of the AC system frequency on their respective sides as the modulation period to generate a multi-level sinusoidal half-wave voltage corresponding to the double frequency of the system; The half-wave commutation units on side 1 and side 2 flip the multi-level sinusoidal half-wave voltages on both sides into full multi-level sinusoidal wave voltages, and connect the AC systems on both sides via filtering elements.

2. A half-wave shaping multi-level AC converter topology according to claim 1, characterized in that: The converters on side 1 and side 2 connected to the AC systems of different frequencies need to be connected through a DC-DC isolation stage to achieve flexible conversion of system voltage, power, and frequency as well as electrical isolation.

3. A half-wave shaping multi-level AC converter topology according to claim 1, characterized in that: The number of submodules of the multilevel shaping unit and the number of DAHB isolation unit modules on both sides are the same, set to N. The number of modules N is configured according to the half-wave peak range of the system AC voltage on both sides and the submodule capacitor voltage.

4. A half-wave shaping multi-level AC converter topology according to claim 1, characterized in that: The DAHB isolation unit uses a high-frequency transformer to achieve electrical isolation while improving system power density.

5. The half-wave commutation unit according to claim 4, characterized in that: The multi-level sinusoidal half-wave is flipped according to the sinusoidal reference waves of the phase voltages on both sides to obtain a complete multi-level sinusoidal wave on the AC side: When the phase voltage sinusoidal reference wave is positive, the commutation switches Q1 and Q4 are turned on, and the AC side outputs the positive half-wave of the multi-level sinusoidal voltage; When the phase voltage sinusoidal reference wave is negative, the commutation switches Q2 and Q3 are turned on, and the AC side outputs the negative half-wave of the multi-level sinusoidal voltage.

6. A control method for a half-wave shaping multi-level AC converter topology suitable for a low-frequency power transmission system, characterized in that: The half-wave shaping multi-level AC converter topology according to any one of claims 1 to 5 comprises: The multi-level shaping units on both sides of the power frequency / low frequency generate a multi-level half-sine wave corresponding to twice the system frequency, which is then flipped into a complete multi-level sine wave by the half-wave commutation unit and connected to the AC system on both sides through the filtering element; The power frequency side adopts fixed module capacitor average voltage and reactive power control, the low frequency side adopts fixed AC voltage control, and the DAHB isolation unit adopts fixed low frequency side module capacitor average voltage control.

Images